1. Field of the Invention
This invention relates to a method and apparatus for testing the efficacy of spotting fluids used for releasing drill pipe which has become stuck during the drilling of a borehole.
2. Discussion of the Art
During the drilling of oil and gas wells, a drilling fluid or "mud" is pumped to the bottom of the well through the hollow drill pipe. Drilling mud cools the drill bit and serves to transport the rock cuttings to the top of the well through the annulus between the drill pipe and the borehole wall. Under some conditions this mud can also form a filter cake on the walls of the borehole, particularly in more permeable rock formations.
The drill pipe can occasionally become stuck to the filter cake, making rotation or removal of the pipe difficult or impossible. When this happens, chemicals known as spotting fluids can be pumped down the drill pipe into the annulus in attempts to free the stuck pipe from the cake.
Many types of spotting fluids are commercially available from well service and supply companies. Choosing an appropriate spotting fluid, however, is not easy because not all fluids are equally effective to release the pipe from the particular filter cake that has resulted from the specific combination of mud types and rock formations in a borehole. The driller has the option of and rock formations in a borehole. The driller has the option of choosing a fluid based upon the driller's past experience, the fluid supplier's recommendations, or a blind guess. These trial and error methods can be expensive due to the high cost of these chemicals.
A second option has been to reproduce the stuck pipe conditions in a laboratory using sections of drill pipe and drilling mud. Candidate spotting fluids are tested to determine which is the most effective. However, such full scale test equipment is expensive and can require a full day for a single test. The expense of idle drilling rigs has driven the search for a simple apparatus and technique that can quickly predict the behavior of one or more spotting fluids when a drill pipe becomes stuck.